A thin film type magnetic recording medium is produced by forming a magnetic layer formed of a ferromagnetic metal or an alloy thereof on a nonmagnetic substrate by means of various methods (for example, sputtering, vapor deposition, and non-electrolytic plating). When practically used, the magnetic recording medium performs contact-slide with respect to a magnetic head at a high speed, so the medium suffers from abrasion damage or is caused to have deterioration of magnetic characteristics in some cases. Accordingly, a protective film and a lubricating layer are provided on the magnetic layer so as to improve abrasion resistance. Hitherto, as a material of the protective film layer, a carbonaceous film, an oxide film such as SiO2, ZrO2, a nitride film, a boride film, and the like have been generally used. In addition, as a material of the lubricating layer, a fluorine-based compound is generally used.
In this way, the lubricating layer of the recording medium is very useful for the purpose of reducing the abrasion damage and the deterioration of magnetic characteristics caused by the contact-slide that occurs between the head and the medium, by means of reducing a coefficient of kinetic friction. On the other hand, particularly, when the film thickness of the lubricating layer is large, adsorption easily occurs between the head and the disk in some cases due to the presence of the lubricating layer. Due to the adsorption, a coefficient of static friction increases, and the head sticks to the disk and does not operate any more in some cases. This adsorption occurs more easily as the medium substrate becomes smoother. In order to heighten surface recording density, it is required to keep the floating height of the head low and speed up the disk rotation. Therefore, since the surface of the medium substrate tends to be smoother, it is important to suppress the adsorption. On the other hand, if the film thickness of the lubricating layer is reduced, though the adsorption does not easily occur, the function of the lubricating layer, that is, the suppression of the abrasion damage and deterioration of the magnetic characteristics caused by the high-speed contact-slide cannot be accomplished in some cases.
Accordingly, various attempts for solving the above problem by carefully selecting a lubricant used for the lubricating layer have been made. For example, compounds disclosed in PTLs 1 and 2 and a lubricant “FOMBLIN Z-DOL” manufactured by Ausimont, Inc. have a CH2OH group at both terminals of a molecule. Therefore, if these are used, it is possible to impart an excellent sliding-resistance characteristic in which the lubricant strongly binds to the surface of the protective film layer.
However, as shown in NPL 1, since the “FOMBLIN Z-DOL” has a bond of an O—CF2—O unit in a molecule, “FOMBLIN Z-DOL” is easily degraded at about 200° C. in the presence of aluminum oxide (α-Al2O3) that is a constituent component of the magnetic head. In addition, at the time of CSS (contact start stop), there are instantaneous local temperature increases up to 90° C. to 450° C. or higher due to the contact between the magnetic recording medium and the magnetic head. Consequently, the aluminum oxide which is a constituent component of the slider portion of the magnetic head serves as a catalyst, and the “FOMBLIN Z-DOL” is degraded. If the lubricant is degraded in this way, the film thickness of the lubricating layer is reduced since the degraded component volatilizes, and the coefficient of kinetic friction increases. As a result, a problem that the abrasion damage and the deterioration of the magnetic characteristics caused by the high-speed contact-slide easily occur arises. Moreover, a part of the degraded component is attached to the magnetic head, which leads to a problem that reproduction output is reduced due to an increase in floating height of the head, and that the head is adsorbed onto the recording medium surface, for example. In this respect, there is a demand for a method that suppresses the contact degradation caused by the magnetic head while maintaining a high substrate-adsorptive property and sliding-resistance characteristic that are advantages of “FOMBLIN Z-DOL”.
As one of the methods, there is a method of using a lubricant “FOMBLIN Z-TETRAOL” manufactured by Ausimont, Inc. having a CH(OH)CH2OH group at both terminals of a molecule. Though including a structure of the O—CF2—O unit in a molecule as in “FOMBLIN Z-DOL”, this lubricant is chemically stable since the contact reaction (degradation) to the aluminum oxide at the slider portion of the head is relieved due to a multidentate structure of a polar functional group. Moreover, the polar functional groups at both terminals have high affinity with a carbon-based or an oxide ceramics-based protective film, which contributes to the improvement of a spin-off property; therefore, “FOMBLIN Z-TETRAOL” has excellent stability.
However, since a large number of the polar functional groups having high affinity with the substrate are present at both terminals, a degree of freedom of the molecules in the lubricating layer is reduced. As a result, a lubricating property becomes insufficient, and the sliding durability deteriorates when “FOMBLIN Z-TETRAOL” is used in combination with a pseudo-contact type head.
In order to solve the above problem, a method of improving the lubricating property and maintaining durability by providing a lubricant component (non-substrate-adsorptive component) (hereinafter, a layer formed of the non-substrate-adsorptive component is referred to as a “non-adsorptive layer”) that can freely move inside the lubricating layer is used (for example, PTL 3).
However, problems that the non-substrate-adsorptive component in the lubricating layer volatilizes easily, and that the reproduction output is reduced due to the increase in the floating height of the head since the non-adsorptive component is attached to the magnetic head when the magnetic head contacts the disk have been reported. In addition, since keeping the floating height of the head low will be promoted in the future to improve the surface recording density, and the lubricating layer is expected to become thinner, the non-adsorptive layer in the lubricating layer is assumed to go against this trend. Consequently, there is a demand for the development of a lubricant that enables the configuration to include only an adsorptive layer while maintaining the high substrate-adsorptive property, sliding-resistance characteristic, and high stability that are advantages of “FOMBLIN Z-TETRAOL”.
Meanwhile, the use of a phosphazene compound, a lubricant composition containing the same, and the use of the lubricant composition as a lubricant for a recording medium have been proposed (PTLs 4 and 5). However, since the adsorptive property or the like of the phosphazene compound with respect to a substrate is insufficient, the phosphazene compound needs to be used in combination with a lubricant such as the FOMBLIN Z-TETRAOL.
Furthermore, various lubricant compositions using a polymer that includes a mesogen structure in a main chain or a side chain and grease compositions have been proposed (for example, Patent PTLs 6, 7, and 8), but whether these compositions are useful as materials used for the lubricating layer for a recording medium has not been clarified.